Electrolytic tin alloy plating solution

ABSTRACT

An electrolytic tin alloy plating solution contains a compound serving as a source of supply of tin ions, a compound serving as a source of supply of silver ions, an oxide of a nitrogen-containing heterocyclic compound, and a flavonoid compound.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2018-195996 filed on Oct. 17, 2018, the entire disclosure of which is incorporated by reference herein.

BACKGROUND ART

The present disclosure relates to an electrolytic tin alloy plating solution, and more particularly, relates to an electrolytic tin alloy plating solution applicable to formation of bumps.

A connection bump is provided on a semiconductor chip. A solder ball or any other suitable structure is used as a connection bump. However, miniaturization of semiconductor chips increases the difficulty in using a known solder ball as the connection bump. Although microballs having a diameter of about 100 μm may also be used, finer design rules have been required. Attention has been given to the formation of a bump plated with tin (Sn) or a Sn alloy (see, for example, Japanese Unexamined Patent Publication No. 2016-106181).

To form a fine bump by plating, a plurality of films with a thickness of several tens of micrometers and independent from one another need to be uniformly deposited in the several-tens-of-micrometer range. In addition, there is a demand for forming a film which has a flat surface as much as possible and which does not easily generate voids after reflow.

SUMMARY OF THE INVENTION

However, known electrolytic tin alloy plating solutions insufficiently reduce voids. With increasing diversification of patterns, there is also a demand for reducing generation of voids as much as possible, even if a bump is formed on a pad with a small diameter.

The present disclosure attempts to provide an electrolytic tin alloy plating solution capable of reducing generation of voids after reflow, even if a bump is formed on a pad with a small diameter.

An electrolytic tin alloy plating solution according to an aspect of the present disclosure contains: a compound serving as a source of supply of tin ions; a compound serving as a source of supply of silver ions; an oxide of a nitrogen-containing heterocyclic compound; and a flavonoid compound.

In the electrolytic tin alloy plating solution according to an aspect, the flavonoid compound may be a flavonoid glycoside.

In the electrolytic tin alloy plating solution according to an aspect, a concentration of the oxide of the nitrogen-containing heterocyclic compound may fall within a range from 0.1 g/L to 10 g/L. A concentration of the flavonoid compound may fall within a range from 0.001 g/L to 20 g/L.

The electrolytic tin alloy plating solution according to an aspect may further contain: a compound serving as a source of supply of copper ions.

The electrolytic tin alloy plating solution according to the present disclosure reduces generation of voids after reflow, even in a bump on a pad with a small diameter.

DESCRIPTION OF EMBODIMENTS

An electrolytic tin alloy plating solution according to this embodiment is used for forming a plating film made of, for example, a tin-silver (Sn—Ag) binary alloy or a tin-silver-copper (Sn—Ag—Cu) ternary alloy. The plating solution contains an oxide of a nitrogen-containing heterocyclic compound and a flavonoid compound. The plating solution containing the oxide of the nitrogen-containing heterocyclic compound and the flavonoid compound, thereby making it possible to form a bump with high surface flatness and less prone to voids after reflow.

Non-limiting examples of the oxide of the nitrogen-containing heterocyclic compound may include 2-methylpyridine N-oxide, nicotinic acid N-oxide, pyridine N-oxide, 4-methylmorpholine N-oxide, 3-acetylpyridine N-oxide, 2-aminopyridine N-oxide, 8-aminoquinoline N-oxide, 2,2′-bipyridyl 1,1′-dioxide, 4-(dimethylamino)pyridine N-oxide hydrate, 4,4′-dimethyl-2,2′-bipyridyl 1-oxide, 3,5-dimethylpyridine N-oxide, 4-(hydroxyamino)quinoline N-oxide, isonicotinic acid N-oxide, 2-hydroxypyridine N-oxide, 3-hydroxypyridine N-oxide, 8-hydroxyquinoline N-oxide, isoquinoline N-oxide, 2,6-lutidine N-oxide, 4-methoxypyridine N-oxide, 3-methylpyridine N-oxide, 4-methylpyridine N-oxide, quinoline N-oxide hydrate, 5,5-dimethyl-1-pyrroline N-oxide, minoxidil, 5-methylpyrazine-2-carboxylic acid 4-oxide, and 3,3,5,5-tetramethyl-1-pyrroline N-oxide. In particular, pyridine N-oxide and nicotinic acid N-oxide largely serve to improve the smoothness and are easily available, and are thus preferably used. These heterocyclic compounds may be used alone or in combination.

To improve the flatness, the concentration of the oxide of the nitrogen-containing heterocyclic compound may be preferably 0.1 g/L or higher, and more preferably 0.5 g/L or higher. In view of the costs, the concentration may be preferably 10 g/L or lower, and more preferably 6 g/L or lower.

The flavonoid compound may be a hydroxylated substance or a glycoside. Examples of the hydroxylated substance may include flavone, 3-hydroxyflavone, 5-hydroxyflavone, 7-hydroxyflavone, chrysin, morin, quercetin, baicalein, Amlexanox, khellin, 6-hydroxyflavone, 7,8-dihydroxyflavone hydrate, 3,4′-dihydroxyflavone, apigenin, 3′,4′-dihydroxyflavone, scutellarein, acacetin, kaempferol hydrate, kaempferide, isorhamnetin, nobiletin, flavanone, 4′-hydroxyflavanone, 3′-hydroxyflavanone, naringenin, hesperetin, dihydromyricetin, (+)-catechin hydrate, (−)-epigallocatechin, α-naphthoflavone, β-naphthoflavone, sodium flavonol-2′-sulfonate hydrate, ipriflavone, pentahydroxyflavone, fisetin, and myricetin. Examples of the glycoside may include rutin hydrate, naringin, methyl hesperidin, baicalin, myricitrin, diosmin, Epmedin C, hesperidin, icariin, neohesperidin, wogonoside, and quercitrin. Above all, a glycoside is preferable since it is applicable at a wide range of current densities, and in particular, rutin hydrate, naringin, methyl hesperidin, and hesperidin are more preferable. These flavonoid compounds may be used alone or in combination.

To improve the flatness, the concentration of the flavonoid compound may be preferably 0.001 g/L or higher, and more preferably 0.01 g/L or higher. In view of the costs, the concentration may be preferably 20 g/L or lower, and more preferably 10 g/L or lower.

In addition to the oxide of the nitrogen-containing heterocyclic compound and the flavonoid compound, the electrolytic tin alloy plating solution according to this embodiment contains a compound serving as a source of supply of tin (Sn) ions and a compound serving as a source of supply of silver (Ag) ions.

Examples of the compound serving as the source of supply of Sn ions include a tin salt. In particular, a first tin salt (tin salt (II)) and a second tin salt (tin salt (IV)) are preferably used.

Non-limiting examples of the first tin salt (tin salt (II)) include tin (II) alkanesulfonate, such as tin (II) methanesulfonate, organic tin (II) sulfonate, such as tin (II) alkanolsulfonate such as tin (II) isethionate, tin (II) sulfate, tin (II) fluoroborate, tin (II) chloride, tin (II) bromide, tin (II) iodide, tin (II) oxide, tin (II) phosphate, tin (II) pyrophosphate, tin (II) acetate, tin (II) citrate, tin (II) gluconate, tin (II) tartrate, tin (II) lactate, tin (II) succinate, tin (II) sulfamate, tin (II) formate, and tin (II) silicofluoride.

Non-limiting examples of the second tin salt (tin salt (IV)) include sodium stannate and potassium stannate.

In particular, tin (II) alkanesulfonate, such as tin (II) methanesulfonate, and organic tin (II) sulfonate, such as tin (II) alkanolsulfonate such as tin (II) isethionate are preferably used.

To reduce burning, the concentration of tin salt as Sn²⁺ is preferably 5 g/L or higher and more preferably 10 g/L or higher. To improve the stability of plating solution and reduce precipitation, the concentration of the tin salt is preferably 100 g/L or lower and more preferably 70 g/L or lower. Such a concentration also helps reduce cost.

Tin salt having a low lead (Pb) concentration of 1.0 ppm or lower may be used as the tin salt. Using tin salt having a low Pb concentration allows the plating solution to have a lower Pb concentration.

Examples of the compound serving as the source of supply of Ag ions may include a silver salt. Non-limiting examples include organic silver sulfonate, silver sulfate, silver fluoroborate, silver chloride, silver bromide, silver iodide, silver oxide, silver phosphate, silver pyrophosphate, silver acetate, silver citrate, silver gluconate, silver tartrate, silver lactate, silver succinate, silver sulfamate, silver formate, and silver silicofluoride. In particular, organic silver sulfonate is preferably used.

In view of the management, the concentration of the compound serving as the source of supply of Ag ions in the form of Ag⁺ is preferably 10 mg/L or higher, and more preferably 20 mg/L or higher. In view of the costs, the concentration is preferably 1000 mg/L or lower, and more preferably 500 mg/L or lower.

The electrolytic tin alloy plating solution according to this embodiment may further contain a compound serving as a source of supply of copper (Cu) ions. Adding the compound serving as the source of supply of Cu ions to the plating solution enables formation of a film of a Sn—Ag—Cu ternary alloy. Examples of the compound serving as the source of supply of Cu ions include a copper salt. Non-limiting examples include organic copper sulfonate, copper sulfate, copper fluoroborate, copper chloride, copper bromide, copper iodide, copper oxide, copper phosphate, copper pyrophosphate, copper acetate, copper citrate, copper gluconate, copper tartrate, copper lactate, copper succinate, copper sulfamate, copper formate, and copper silicofluoride. In particular, organic copper sulfonate is preferably used.

In view of the management and the concentration distribution, the concentration of the compound serving as the source of supply of Cu ions in the form of Cu⁺ is preferably 10 mg/L or higher, and more preferably 50 mg/L or higher. In view of the solution stability, the concentration is preferably 5000 mg/L or lower, and more preferably 2000 mg/L or lower.

The electrolytic tin alloy plating solution according to this embodiment may contain any one of an inorganic acid, an organic acid, and soluble salts of these acids. Adding the acid or the soluble salt to the plating solution allows the pH of the surface of a plated object and the pH of the surface of the Sn—Ag alloy forming the plated film to be kept constant, thus providing a uniform surface potential. This can retard the eutectoid reaction of Pb.

Non-limiting examples of the acid or the soluble salt of the acid include a sulfuric acid, a hydrochloric acid, a nitric acid, a phosphoric acid, a sulfamic acid, an organic sulfonic acid (an alkanesulfonic acid, such as a methanesulfonic acid, or an alkanolsulfonic acid, such as an isethionic acid), and a carboxylic acid (an aromatic carboxylic acid, an aliphatic saturated carboxylic acid, or an amino carboxylic acid). If necessary, a neutralized salt of any one of these soluble salts may be used. In particular, a methanesulfonic acid is preferably used for ease of handling.

To improve the stability of the plating solution and reduce the precipitation, the concentration of the acid or the soluble salt of the acid is preferably 35 g/L or higher, and more preferably 50 g/L or higher. This concentration also helps substantially prevent an appropriate Pb deposition potential. In view of the costs, the concentration is preferably 140 g/L or lower, and more preferably 120 g/L or lower.

The electrolytic tin alloy plating solution according to this embodiment may contain a surfactant. One or more selected from an anionic surfactant, a cationic surfactant, and a nonionic surfactant may be used as the surfactant. In particular, a nonionic surfactant is preferably used, and an alkylene oxide-based surfactant is more preferably used. Adding the surfactant to the plating solution allows the surface of a plated object and the surface of a Sn crystal serving as a film to have a uniform current density, thus maintains a uniform deposition potential at the surface. This can retard the eutectoid reaction of Pb.

Non-limiting examples of the alkylene oxide-based surfactant include polyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene alkyl amine, polyoxyethylene alkyl amide, polyoxyethylene fatty acid ester, polyoxyethylene polyhydric alcohol ether, an ethylene oxide-propylene oxide block copolymer compound, an ethylene oxide-propylene oxide random copolymer compound, and a propylene oxide polymer compound. In particular, polyoxyethylene alkyl phenyl ether is preferably used.

The concentration of the surfactant is preferably 0.05 g/L or higher, and more preferably 0.5 g/L or higher. Even if plating is performed at a high current density to shorten the plating time, the surfactant having such a concentration or higher can reduce burning at an area having a high current density. To reduce color irregularities arising from blackening of the plated film, the concentration of the surfactant is preferably 100 g/L or lower.

The electrolytic tin alloy plating solution according to this embodiment contains an acid or a soluble salt thereof and a surfactant. The acid or the soluble salt thereof is more preferably one or more acids or soluble salts thereof selected from a sulfuric acid, a hydrochloric acid, a nitric acid, a phosphoric acid, a sulfamic acid, an organic sulfonic acid, a carboxylic acid, or salts of these acids. The surfactant is more preferably selected from one or more surfactants selected from an anionic surfactant, a cationic surfactant, and a nonionic surfactant.

The electrolytic tin alloy plating solution according to this embodiment may contain an organic solvent, an antioxidant, and a chelating agent. Non-limiting examples of the organic solvent include monohydric alcohols, such as methanol and 2-propanol, and dihydric alcohols, such as ethylene glycol, diethylene glycol, and triethylene glycol. Non-limiting examples of the antioxidant include catechol, hydroquinone, 4-methoxyphenol, and ascorbic acid. Non-limiting examples of the chelating agent include oxalic acid, succinic acid, malonic acid, glycolic acid, gluconic acid, gluconolactone, glycine, ethylenediamine-acetic acid, pyrophosphoric acid, and tripolyphosphoric acid.

To form a plated film using the electrolytic tin alloy plating solution according to this embodiment, the pH of the plating solution is preferably strongly acidic. The temperature at which the plated film is formed should not be specifically limited. However, the temperature is preferably from 25° C. to 40° C. The current density at the formation of the plating film is preferably from 1 A/dm² to 20 A/dm², and more preferably from 2 A/dm² to 10 A/dm².

The electrolytic tin alloy plating solution according to the present disclosure can be used, for example, to form plated bumps on a semiconductor chip. To form a plated bump, a plating film with a predetermined size may be formed in a predetermined position, which may be followed by a reflow process. The reflow process should not be specifically limited, but may be performed using a known reflow apparatus.

EXAMPLES

The electrolytic tin alloy plating solution according to the present disclosure will now be described in more detail with reference to examples. The following examples are illustrative, and are not intended to limit the present disclosure.

<Formation of Plated Film>

An electrolytic tin alloy plating solution containing a predetermined composition was used at a temperature of 30° C. and a current density of 4 A/dm² to form bumps made of a tin alloy plating film on the surface of pads made of electric copper. The diameters of the pads were 20 μm, 50 μm, 80 μm, and 100 μm.

<Evaluation of Void>

The obtained bumps were reflowed at 260° C., and then the presence or absence of voids was evaluated by an X-ray nondestructive testing system (XD7600NT Diamond FP, manufactured by Nordson DAGE). The X-ray nondestructive testing system had a tube voltage of 60 kV and an output of 1.5 W.

Example 1

As the oxide of the nitrogen-containing heterocyclic compound, pyridine N-oxide was used at a concentration of 5 g/L. As the flavonoid compound, naringin was used at a concentration of 0.3 g/L. As the source of supply of Sn ions, tin (II) alkanesulfonate in the form of Sn²⁺ was added at a concentration of 60 g/L. As the source of supply of Ag ions, silver alkanesulfonate in the form of Ag⁺ was added at a concentration of 0.5 g/L. As the organic acid, methanesulfonic acid was added at a concentration of 70 g/L. A nonionic surfactant was also added at a concentration of 20 g/L. The plating solution had a temperature of 30° C. and a current density of 4 A/dm².

No void was observed after reflow in any of the obtained bumps which were made of the Sn—Ag alloy plated film on the pad with any of the above diameters.

Example 2

A Sn—Ag alloy plated film was formed in the same manner as in Example 1, except that nicotinic acid N-oxide was used as the oxide of the nitrogen-containing heterocyclic compound at a concentration of 1 g/L. No void was observed after reflow in any of the obtained bumps which were made of the Sn—Ag alloy plated film on the pad with any of the above diameters.

Third Example

A Sn—Ag alloy plated film was formed in the same manner as in Example 1, except that pyridine N-oxide was used as the oxide of the nitrogen-containing heterocyclic compound at a concentration of 3 g/L and methyl hesperidin was used as the flavonoid compound at a concentration of 5 g/L. No void was observed after reflow in any of the obtained bumps which were made of the Sn—Ag alloy plated film on the pad with any of the above diameters.

Example 4

A Sn—Ag alloy plated film was formed in the same manner as in Example 1, except that nicotinic acid N-oxide was used as the oxide of the nitrogen-containing heterocyclic compound at a concentration of 2 g/L and rutin hydrate was used as the flavonoid compound at a concentration of 3 g/L. No void was observed after reflow in any of the obtained bumps which were made of the Sn—Ag alloy plated film on the pad with any of the above diameters.

Example 5

A Sn—Ag alloy plated film was formed in the same manner as in Example 1, except that nicotinic acid N-oxide was used as the oxide of the nitrogen-containing heterocyclic compound at a concentration of 3 g/L and hesperidin was used as the flavonoid compound at a concentration of 0.5 g/L. No void was observed after reflow in any of the obtained bumps which were made of the Sn—Ag alloy plated film on the pad with any of the above diameters.

Example 6

A Sn—Ag alloy plated film was formed in the same manner as in Example 1, except that nicotinic acid N-oxide was used as the oxide of the nitrogen-containing heterocyclic compound at a concentration of 1 g/L and methyl hesperidin was used as the flavonoid compound at a concentration of 5 g/L. No void was observed after reflow in any of the obtained bumps which were made of the Sn—Ag alloy plated film on the pad with any of the above diameters.

Example 7

A Sn—Ag alloy plated film was formed in the same manner as in Example 1, except that pyridine N-oxide was used as the oxide of the nitrogen-containing heterocyclic compound at a concentration of 3 g/L and hesperidin was used as the flavonoid compound at a concentration of 0.5 g/L. No void was observed after reflow in any of the obtained bumps which were made of the Sn—Ag alloy plated film on the pad with any of the above diameters.

First Comparative Example

A Sn—Ag alloy plated film was formed in the same manner as in Example 1, except that no oxide of a nitrogen-containing heterocyclic compound is added and baicalin was used as the flavonoid compound at a concentration of 1 g/L. No void was observed after reflow in the obtained bump, which was made of the Sn—Ag alloy plated film, on the pad with the diameter of 100 μm. However, voids were observed after reflow in the obtained bumps on the pads with the diameters of 80 μm, 50 μm, and 20 μm.

Second Comparative Example

A Sn—Ag alloy plated film was formed in the same manner as in Example 1, except that no oxide of a nitrogen-containing heterocyclic compound was added and naringin was used as the flavonoid compound at a concentration of 5 mg/L. No void was observed after reflow in the obtained bumps, which were made of the Sn—Ag alloy plated film, on the pads with the diameters of 100 μm and 80 μm. However, voids were observed after reflow in the obtained bumps on the pad with the diameters of 50 μm and 20 μm.

Third Comparative Example

A Sn—Ag alloy plated film was formed in the same manner as in Example 1, except that no oxide of a nitrogen-containing heterocyclic compound was added and methyl hesperidin was used as the flavonoid compound at a concentration of 3 mg/L. No void was observed after reflow in the obtained bumps, which were made of the Sn—Ag alloy plated film, on the pads with the diameters of 100 μm and 80 μm. However, voids were observed after reflow in the obtained bumps on the pad with the diameters of 50 μm and 20 μm.

Fourth Comparative Example

A Sn—Ag alloy plated film was formed in the same manner as in Example 1, except that pyridine N-oxide was used as the oxide of the nitrogen-containing heterocyclic compound at a concentration of 6 g/L and no flavonoid compound was added. No void was observed after reflow in the obtained bumps, which were made of the Sn—Ag alloy plated film, on the pads with the diameters of 100 μm and 80 μm. However, voids were observed after reflow in the obtained bumps on the pad with the diameters of 50 μm and 20 μm.

Fifth Comparative Example

A Sn—Ag alloy plated film was formed in the same manner as in Example 1, except that 5,5-dimethyl-1-pyrroline N-oxide was used as the oxide of the nitrogen-containing heterocyclic compound at a concentration of 3 g/L and no flavonoid compound was added. No void was observed after reflow in the obtained bump, which was made of the Sn—Ag alloy plated film, on the pad with the diameter of 100 μm. However, voids were observed after reflow in the obtained bumps on the pads with the diameters of 80 μm, 50 μm, and 20 μm.

TABLE 1 Examples 1 2 3 4 5 6 7 Oxide of Pyridine Nicotinic Pyridine Nicotinic Acid Nicotinic Acid Nicotinic Acid Pyridine Nitrogen-Containing N-Oxide Acid N-Oxide N-Oxide N-Oxide N-Oxide N-Oxide Heterocyclic Compound N-Oxide Concentration (g/L) 5   1   3 2 3   1 3   Flavonoid Compound Naringin Naringin Methyl Rutin Hydrate Hesperidin Methyl Hesperidin Hesperidin Hesperidin Concentration (g/L) 0.3 0.3 5 3 0.5 5 0.5 Generation φ 100 μm  None None None None None None None of Voids φ 80 μm None None None None None None None φ 50 μm None None None None None None None φ 20 μm None None None None None None None

TABLE 2 Comparative Examples 1 2 3 4 5 Oxide of Nitrogen-Containing — — — Pyridine 5,5-Dimethyl-1-Pyrroline Heterocyclic Compound N-Oxide N-Oxide Concentration (g/L) — — — 6 3 Flavonoid Compound Baicalin Naringin Methyl — — Hesperidin Concentration (g/L) 1 0.005 0.003 — — Generation of φ 100 μm  None None None None None Voids φ 80 μm Generated None None None Generated φ 50 μm Generated Generated Generated Generated Generated φ 20 μm Generated Generated Generated Generated Generated

The results of the examples and comparative examples are collectively shown in Tables 1 and 2. If the plating solution contains both the oxide of the nitrogen-containing heterocyclic compound and the flavonoid compound, no void was generated after reflow in the bumps even if the pad has a small diameter. However, if the plating solution contains only one, voids were generated after reflow in the bumps on the pads with small diameters.

The electrolytic tin alloy plating solution of the present disclosure reduces generation of voids after reflow even if the pad has a small diameter, and is thus useful in formation of bumps. 

What is claimed is:
 1. An electrolytic tin alloy plating solution, comprising: a compound serving as a source of supply of tin ions; a compound serving as a source of supply of silver ions; an oxide of a nitrogen-containing heterocyclic compound; and a flavonoid compound.
 2. The electrolytic tin alloy plating solution according to claim 1, wherein the flavonoid compound is a flavonoid glycoside.
 3. The electrolytic tin alloy plating solution according to claim 1, wherein a concentration of the oxide of the nitrogen-containing heterocyclic compound falls within a range from 0.1 g/L to 10 g/L, and a concentration of the flavonoid compound falls within a range from 0.001 g/L to 20 g/L.
 4. The electrolytic tin alloy plating solution according to claim 1, further comprising a compound serving as a source of supply of copper ions.
 5. The electrolytic tin alloy plating solution according to claim 1, wherein the oxide of the nitrogen-containing heterocyclic compound is one or more selected from 2-methylpyridine N-oxide, nicotinic acid N-oxide, pyridine N-oxide, 4-methylmorpholine N-oxide, 3-acetylpyridine N-oxide, 2-aminopyridine N-oxide, 8-aminoquinoline N-oxide, 2,2′-bipyridyl 1,1′-dioxide, 4-(dimethylamino)pyridine N-oxide hydrate, 4,4′-dimethyl-2,2′-bipyridyl 1-oxide, 3,5-dimethylpyridine N-oxide, 4-(hydroxyamino)quinoline N-oxide, isonicotinic acid N-oxide, 2-hydroxypyridine N-oxide, 3-hydroxypyridine N-oxide, 8-hydroxyquinoline N-oxide, isoquinoline N-oxide, 2,6-lutidine N-oxide, 4-methoxypyridine N-oxide, 3-methylpyridine N-oxide, 4-methylpyridine N-oxide, quinoline N-oxide hydrate, 5,5-dimethyl-1-pyrroline N-oxide, minoxidil, 5-methylpyrazine-2-carboxylic acid 4-oxide, and 3,3,5,5-tetramethyl-1-pyrroline N-oxide.
 6. The electrolytic tin alloy plating solution according to claim 1, wherein the flavonoid compound is one or more selected from flavone, 3-hydroxyflavone, 5-hydroxyflavone, 7-hydroxyflavone, chrysin, morin, quercetin, baicalein, Amlexanox, khellin, 6-hydroxyflavone, 7,8-dihydroxyflavone hydrate, 3,4′-dihydroxyflavone, apigenin, 3′,4′-dihydroxyflavone, scutellarein, acacetin, kaempferol hydrate, kaempferide, isorhamnetin, nobiletin, flavanone, 4′-hydroxyflavanone, 3′-hydroxyflavanone, naringenin, hesperetin, dihydromyricetin, (+)-catechin hydrate, (−)-epigallocatechin, α-naphthoflavone, β-naphthoflavone, sodium flavonol-2′-sulfonate hydrate, ipriflavone, pentahydroxyflavone, fisetin, and myricetin.
 7. The electrolytic tin alloy plating solution according to claim 2, wherein the flavonoid compound is one or more selected from rutin hydrate, naringin, methyl hesperidin, baicalin, myricitrin, diosmin, Epmedin C, hesperidin, icariin, neohesperidin, wogonoside, and quercitrin. 